This invention relates generally to computed tomography (CT) imaging methods and apparatus, and more particularly to methods and apparatus for reconstructing images of objects from partial scans.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Ideally, in cardiac CT reconstruction, scanner speed would be matched to a heart rate of a patient so that as many contiguous (or slightly overlapped) sectors would be acquired as there are measuring rows in the imaging system. For a four-slice system, it is desirable to subdivide a range of source angles into four sectors that, when combined together, allow image reconstruction from data essentially acquired at the same cardiac phase. Although a minimum source angle span of 180+fan-angle (.pi.+2.GAMMA.) is sought for reasons of adequate temporal resolution, it is not always possible to achieve this minimum due to the limited number of scanner speeds and wide range of patient heart rates. It would therefore be desirable to provide means and apparatus to provide image quality optimization for a given time resolution with full utilization of patient dose.
Due to the continuous range of patient cardiac motion, and the fixed number of scanner rotation speeds, cardiac sector reconstruction, in practice, leads to a usable data set that encompass more views than a minimum number required for half-scan reconstruction. Patient dose and image noise considerations indicate that it would be desirable to provide means and apparatus for a partial scan reconstruction in which any helical pitch faster than a high-quality (HQ) mode could be directly reconstructed.